This matches the API of Account::authenticate. The only caller to this
API is the passwd utility, which already has the new password stored as
a SecretString.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
If a user is missing from /etc/shadow, we used to just allow anyone to
authenticate as that user without a password.
With this patch, authentication will instead always fail.
Now that we've moved to atomic replacement of these files when altering
them, we don't need to keep them open for the lifetime of Core::Account
so just simplify this and close them when they are not needed.
